Hitherto, soldering has been used widely as a connecting manner with which an electronic part is mounted to a substrate. For such soldering, various processes are employed such as a solder bump formation process in which metal bumps as connection electrodes of the electronic part are formed of a solder, a solder pre-coating process in which solder layers are formed on electrode surfaces of the substrate, and the like. Recently, in the soldering, a so-called lead-free solder which contains almost or substantially no harmful lead has got to be used in the above mentioned soldering processes from a viewpoint of the environmental conservation.
Since the lead-free solder is greatly different from the conventionally used solder in its components as well as its composition, the conventional flux which is used in the conventional soldering process cannot be used with the lead-free solder. That is, the conventional flux has an activity which is not sufficient for removing an oxide film on a surface of the lead-free solder, so that it is not easy for the conventional flux to ensure a good soldering connection. Therefore, a flux of which activity is increased has been developed for the lead-free solder. See Patent References 1, 2 and 3 as mentioned below.
Also, it has been proposed that a flux contains a large amount of a metal powder for the purpose of providing a metal paste which can be transferred to a solder ball in an sufficient amount. See Patent Reference 4 as mentioned below.
Patent Reference 1: JP Patent Kokai Publication No. 2002-1581
Patent Reference 2: JP Patent Kokai Publication No. 2000-135592
Patent Reference 3: JP Patent Kokai Publication No. 254184/1999
Patent Reference 4: JP Patent Kokai Publication No. 2000-31210